Syphilis is a sexually transmitted disease (STD) caused by the spirochete bacterium Treponema pallidum. Over 100,000 cases of adult syphilis are reported worldwide each year. In addition, the disease is transmitted congenitally and affects 3,000 or more infants annually. The course of syphilis infection spans many years and may lead to a variety of clinical presentations, which are characterized by four stages.
The primary stage of syphilis infection occurs 10-100 days after bacterial infection, and is characterized by the appearance of one or more chancres (red, bloodless, painless ulcers typically less than 1 cm in diameter). The chancres may appear on the genitalia or elsewhere on the body. A chancre lasts 3-6 weeks and heals without treatment, leaving a small scar. Infected persons are contagious during this stage.
The secondary stage of syphilis infection is characterized by rash-like skin lesions that can cover part or all of the body. The skin lesions are generally painless and appear 1-6 months after the onset of the initial chancre(s). The skin lesions can resemble warts, pustules, or ulcers. Left untreated, they heal in 2-12 weeks without scarring. Fever, sore throat, weakness, weight loss, swelling of the lymph nodes, and loss of the eyelashes and/or part of the eyebrows can also occur during this stage of infection. In addition, the symptoms may progress to meningovascular syphilis, which is characterized by inflammation of the covering of the brain and spinal cord and/or changes in the vasculature of the brain. Infected persons are also contagious in the secondary phase.
The next stage of this disease is latent syphilis or the hidden stage. During this stage, the infected person appears to have recovered and is asymptomatic. This stage lasts for life in approximately two-thirds of persons who are not treated for syphilis. During the first year of latency, relapses of secondary stage symptoms may occur. Except during a relapse, infected persons are not contagious during this latent stage; however, children born to latently infected mothers within four years of the appearance of the primary chancre may contract congenital syphilis.
Tertiary or late syphilis is the final stage of untreated infection. This stage may occur as early as one year after infection or anytime thereafter with 10 to 20 years being most common. Benign syphilis, characterized by lesions called gummas, can occur in the bone, skin, and internal organs. Death is rare, but severe disfigurement and pain can occur. Cardiovascular syphilis is characterized by aortic aneurisms as well as other cardiovascular problems and frequently results in death. Neurologic involvement may occur in the early stages of syphilis as well as manifest as late stage symptoms. In the late stage disease, neurosyphilis may be asymptomatic or the patient may have severe neurologic problems such as possible dementia, insanity, impairment of mobility, blindness, deafness, or even death.
The immune response in syphilis involves production of (i) treponemal antibodies, which are specific for T. pallidum antigens, and (ii) anti-lipoidal antibodies, which recognize lipoidal material released from damaged host cells, lipoprotein-like material and possibly cardiolipin released from the treponemes. The mainstay of syphilis screening and diagnosis is serological testing for either or both of these two types of antibodies.
Tests for anti-lipoidal antibodies (often called “non-treponemal tests”) are typically based on an antigen composed of naturally occurring cardiolipin, cholesterol and lethicin. The widely used non-treponemal tests (e.g., Venereal Disease Research Laboratory (VDRL) test and Rapid Plasma Reagin (RPR) test) monitor, either microscopically (e.g., VDRL test) or macroscopically (e.g., RPR test), the formation of a flocculent comprised of antigen-antibody complexes. Non-treponemal tests have the advantage of being widely available, inexpensive and convenient to perform on large numbers of specimens. Moreover, because anti-lipoidal antibody titers decrease with successful treatment for syphilis, eventually disappearing in most patients, while treponemal antibodies titers remain high for years or even a lifetime, non-treponemal tests are considered the better choice for monitoring treatment or testing for reinfection.
Treponemal tests are based on antigens derived from T. pallidum and include the T. pallidum particle agglutination (TP-PA), the fluorescent treponemal antibody-absorbed test (FTA-ABS) and enzyme immunoassays. Treponemal tests are used primarily to verify reactivity in non-treponemal tests. The treponemal test may also be used to confirm a clinical impression of syphilis in which the non-treponemal test is nonreactive. Treponemal tests are technically more difficult, time consuming, and expensive to perform and cannot be used to monitor treatment because the test will remain reactive for years or a lifetime in approximately 85% of persons successfully treated for syphilis.
Each of the above-described antibody tests is performed using a serum sample that is obtained in a clinical setting and sent to a laboratory for analysis. Therefore, test results are typically not available for several days after the sample is collected. Because of the frequent difficulty of tracing patients with STDs, the development of a rapid, point-of-care test is needed to aid the clinician in making a judgment, preferably on the day of testing.
Immunoassay devices (such as test strips, flow-through devices, or lateral flow devices), which offer rapid, on-site results, are available to qualitatively test serum levels of treponemal antibodies (e.g., DiaSys Corporation; ACON Laboratories, Inc.; Biokit, S.A.; Genix Technology; Standard Diagnostics; Cortez Diagnostics, Inc.; and Phoenix Bio-Tech Corp). However, analogous tests for anti-lipoidal antibodies have been more difficult to develop at least in part because the hydrophobic antigens of the anti-lipoidal antibodies (e.g., VDRL, USR or RPR antigens, or cardiolipin) resist attachment to solid supports, which is one element of an immunoassay device.
According to some experts, syphilis detection would be further aided by a combination of a non-treponemal test and a treponemal test for screening and diagnostic purposes. This is an approach advocated by the World Health Organization, Treponemal Infections, Technical Report Series 674, Geneva: WHO, 1982. An easy-to-use, rapid, point-of-care test capable of concurrently detecting both non-treponemal and treponemal antibodies would help address this long-felt need.